NCEA EXAM QUESTION: Describing and Explaining shapes and bond angles. In your answer, you must make reference to the arrangement of electrons.
The Lewis structure for a molecule containing atoms of boron, oxygen, and hydrogen, is shown below.
The following table describes the shapes around two of the atoms in the molecule above. Complete the table with the approximate bond angles
ii) The bond angles x and y in the molecule above are different. Elaborate on why the bond angles are
different. In your answer you should include:
• factors which determine the shape around the:
- B atom for bond angle x
- O atom for bond angle y
• reference to the arrangement of electrons around the B and O atoms
Boron and phosphorus both bond with three fluorine atoms to form BF3 and PF3. However, the molecules have different shapes and bond angles. The following table shows the Lewis structures for the molecules BF3 and PF3.
Explain why these molecules have different shapes and bond angles.
In your answer include:
• the shapes of BF3 and PF3
• factors that determine the shape of each molecule
• the approximate bond angle in BF3 and PF3
• justification of your chosen bond angles for each molecule.
The following table shows the Lewis structures and bond angles for the molecules SO2 and H2CO.
Explain why these molecules have different shapes, but have the same approximate bond angle. In your answer you should include:
• the shapes of SO2 and H2CO
• factors which determine the shape of each molecule
• an explanation of why the approximate bond angle is the same by referring to the arrangement of electrons for each molecule.
The shapes of the two molecules SO2 and H2S are shown in the diagram below. The shape of both molecules is described as bent. Discuss.
Lewis structures for two molecules are given below.
For each molecule, name the shape of the molecule and give a reason for your answer.
The shape of both molecules can be described as bent. However, these molecules do not have the same bond angle.
Discuss why these molecules have different bond angles. Your answer must include:
factors which determine the shape of each molecule
the approximate bond angle for each molecule.
The Lewis structures of the molecules NCl3 and SO3 are given below.
Discuss the shapes and bond angles of these two molecules. For each molecule:
• name the shape
• determine the bond angle
• justify your answers.
Lewis structures for TWO molecules are given below. For each molecule :
• name the shape
• justify your answer.
For each of the molecules in the table, name the shape and explain why it has the shape you have identified.
(i) CH3Cl
(ii) NCl3
(iii) CH2O
Molecules of water (H2O) and ozone (O3) each contain 3 atoms and both the molecules are bent. However, the bond angle in H2O significantly smaller than the bond angle in O3. Discuss.
NCEA EXAM QUESTION For each molecule listed in the table above, explain the reason for your choice
Elements M and X form a compound MX2. Atoms of element X have a higher electronegativity value than atoms of element M, therefore the M–X bonds are polar.
Depending on what elements M and X are, molecules of the compound formed will be polar or non-polar.
State the most likely shape(s) of the molecule if it is i) polar or ii) non-polar
Justify your answer and draw diagrams of the possible molecules with dipoles labelled. You do not need to identify what elements M and X are.
NCEA EXAMINATION QUESTION: This question is based on a experiment that you should have carried out in class
...you missed your practical lessons? This virtual experiment may help, click on the image below
Two burettes are set up. One burette contains water (a polar liquid) and the other contains cyclohexane (a non-polar liquid). The liquid is allowed to run from each burette in a steady stream. A charged plastic ruler is then placed near the stream of each liquid.Describe and Explain what will be seen when the charged ruler is placed near the stream of each liquid.
NCEA EXAM QUESTIONS: Justify each of the properties of the following substances
Use your knowledge of structure and bonding to explain the dissolving process of sodium chloride in water. Support your answer with an annotated (labelled) diagram.
Silicon dioxide has a melting point of 1770°C.Explain why silicon dioxide has a high melting point by referring to the particles and the forces between the particles in the solid.
see below a copy of a related question that was in the
The diagrams below show structural representations of the two solids ice, H2O, and sodium chloride, NaCl. Ice melts at 0°C and sodium chloride melts at 801°C.
On each diagram above, circle ONE of the forces of attraction which must be overcome for the substance to melt. Give a reason for your choice.
Discuss the reasons why the following two carbon-containing compounds (methane and calcium carbide) have different melting points. The melting points are given in the table below.
There is no Crystal Ball activity here as once you've completed the student activity above and marked them you'll be sweet with Explaining properties of solids.
NCEA EXAM QUESTIONS: Compare and contrast the properties of the following substances
Additional questions on Comparing and Contrasting substances
Using your knowledge of structure and bonding, explain why, although both graphite and copper are good conductors of electricity, copper is suitable for electrical wires, but graphite is not.
Choose the term (endothermic or exothermic) that best describes this process.
Choose the phrase that best describes what you would observe happening to the beaker during this process (gets colder, stays the same, gets warmer) Explain your choice.(b) Glucose is an important source of energy in our diet. The equation below shows the combustion of glucose to form carbon dioxide and water.C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(ℓ) Δr H° = –2 820 kJ mol–1
(i) Choose the term (endothermic orexothermic) that best describes this process. Give a reason for your choice.
(ii) Females who are moderately active need 9 800 kJ of energy per day. Calculate the number of moles of glucose that would provide this daily energy requirement.
(c) Many portable BBQ and camping gas canisters contain butane, C4H10. Butane is a gas at room temperature, and has a boiling point of – 0.5°C. The gas canisters contain both gas and liquid butane. As the gaseous butane is used, some of the liquid evaporates.
Choose the term (endothermic orexothermic) that best describes this process.
Give a reason for your choice, and use your knowledge of structure and bonding, and energy changes, to explain the changes occurring as the liquid evaporates.
a) Some Bunsen burners use methane gas, CH4, as a fuel. The reaction for the combustion of methane in a Bunsen burner is shown in Equation One below.
When this reaction occurs, bonds are broken and bonds are formed. State which bonds are broken and which bonds are formed during the reaction
b)The equation for water boiling at 100°C is shown below in Equation Two.
Equation Two: H2O(ℓ) → H2O(g) ΔrH = 40.7 kJ mol–1
Explain why this equation is endothermic. You should relate the energy changes that are occurring to the specific bonds being broken or formed.
a) Ice melting to form water can be represented by the following equation: State whether the reaction is Endothermic or Exothermic and give reasons for your choice
b) The reaction between hydrogen gas and oxygen gas to form water in the gaseous state can be represented by:
When this reaction occurs, bonds are broken and bonds are formed, state which bonds are broken and which bonds are formed. The bond breaking and bond forming processes above can be described as EITHER exothermic OR endothermic. State which process is exothermic and which process is endothermic and Explain your answer
(a) For the reaction 4NH3(g) + 5O2(g) --> 4NO(g) + 6H2O(g), the enthalpy of reaction is ΔrH = –950 kJ mol–1.
The reverse reaction is 4NO(g) + 6H2O(g) --> 4NH3(g) + 5O2(g)
Two energy diagrams are shown below. One is for the forward reaction and one is for the same reaction in the reverse direction.
i) For each diagram, indicate whether the diagram is for an exothermic reaction or an endothermic reaction
ii) For each diagram, indicate whether the diagram is for a forward or reverse reaction
iii) On the diagrams for both reactions, also label the following: • enthalpy of reactionΔrH and • Activation Energy, Ea.
iv) Determine the value for the enthalpy of reaction (ΔrH ) for the reaction shown in Diagram Two. Explain how you determined this value.
Justify which fuel, methanol or ethanol, will produce more heat energy when 345 g of each fuel is combusted in excess oxygen.
M(CH3OH) = 32.0 g mol–1 M(C2H5OH) = 46.0 g mol–1
1) The equation below shows the combustion of butane. C4H10(g) + 13/2 O2(g) → 4CO2(g) + 5H2O(g)
When 100 g of butane undergoes combustion, 4 960 kJ of energy is released. Calculate the enthalpy change when 1 mole of butane undergoes combustion. M(C4H10) = 58.1 g mol–1.
2) The iron oxides Fe3O4 and Fe2O3 react with aluminium as shown below.
Determine which of the fuels, hydrogen or methane, provides the most energy per gram of fuel burned. Justify your answer with calculations.
When a 12.2 g sample of ammonia is burned, 275 kJ of energy is released. Calculate the energy released for the reaction below, when four moles of ammonia are burned.
4NH3(g) + 3O2(g) → 2N2(g) + 6H2O(ℓ)
The equation for the reaction between calcium oxide, CaO, and water can be represented as Calculate the mass of calcium oxide required to release 287 kJ of energy.
CaO(s) + H2O(ℓ) → Ca(OH)2(aq) ΔH = –82.0 kJ mol–1
When an 18.4 g sample of ethanol is burned, 546 kJ of energy is released. Determine the enthalpy change, ΔrH, for the reaction when one mole of ethanol is burned.
C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l)
(1) 29.6 g of sodium hydroxide was dissolved in water and excess hydrochloric acid was added. Using the temperature increase and the heat capacity of water, it was calculated that 43.5 kJ of heat was released.
(i) Determine the enthalpy change, ΔrH, for the following reaction:
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
(ii) What mass of sodium hydroxide is required to produce 150 kJ of energy?
(2) The principle of a fireworks-type explosion can be demonstrated by igniting a sucrose jellybaby with sodium chlorate, NaClO3. The equation for the explosion reaction is:
(i) 1.00 litre of octane contains 6.12 moles of the fuel. Calculate the energy released when 1 litre of the fuel is burnt.
Using hydrogen gas (H2) as a fuel for cars, rather than octane, is often viewed as better for the environment.
(ii) Calculate the mass of H2 required to produce the same amount of energy as 1.00 litre of octane, as calculated in part (b) above. State your answer to 3 significant figures. H2(g) + 1/2 O2(g) → H2O(g) ΔrH= -286 kJmol-1
NCEA EXAM QUESTIONS: Carry out the following Bond Energy Calculations
Use the Bond Enthalpy values provided in the table below to answer the questions (referenced from previous NCEA Exams)
Hydrogen gas, H2(g), reacts with oxygen gas, O2(g), as shown by the following equationH2(g) + ½ O2(g) → H2O(g) Δr Ho = –242 kJ mol–1Given the average bond enthalpies in the table below, calculate the average bond enthalpy of the O – H bond in H2O.
Chlorine reacts with methane to form chloromethane and hydrogen chloride, as shown in the equation below.
CH4(g) + Cl2(g) → CH3Cl(g) + HCl(g)
Use the following bond enthalpies to calculate Δr H° for this reaction.
a)The equation for the combustion of propan-1-ol is:
The bond enthalpies for the carbon to oxygen bonds in CO2 and CO are different.
i) Use the bond enthalpies and the enthalpy of the reaction to calculate the bond enthalpy of the carbon to oxygen bond in CO. Why are bond enthalpy values always positive? Explain the difference between the following bond enthalpies.
ii) Why are bond enthalpy values always positive?
iii) Explain the difference between the C – O and C = O enthalpies
A fuel cell, such as that used on a space-craft, is similar to a battery. An example is the fuel cell that ‘burns’ hydrogen and oxygen to produce water and energy.
GRAB YOUR CALCULATOR, NCEA RESOURCE BOOK AND TWO PENS (ONE SPARE!)FIND A QUIET DESK FOR 1 HOURNO MOBILES, NO LAPTOPS, NO DISTRACTIONSDO YOUR BEST EFFORT AT ONE OF THE EXAMS BELOW
The following table describes the shapes around two of the atoms in the molecule above. Download this FREE preview of a BRILLIANT Level 2 Chemistry workbookBeginning Chemistry (2012) Anne Wignall and Terry Wales, Pearson PublishersAnswers included at the back of the book too (Thank you!) Download this FREE live preview of an INCREDIBLY student friendly Level 2 Chemistry workbookandREALLY student friendly Level 2 Chemistry Revision workbookSciPAD (2015) Jason Rendle, Silverback Academic Media
Complete the table with the approximate bond angles x and y.
2.4 AS 91164 Demonstrate understanding of bonding, structure, properties & energy changes (5 credits)
LEWIS DIAGRAMS
NCEA PAST EXAM QUESTION: Draw the Lewis structure (electron dot diagram) for each molecule
The table below provides an overview of the molecules and year that they appeared in the NCEA Exams
Download and print the Lewis.doc below which contains the molecules (as shown above) with space to draw the Lewis diagrams. Check your answers.
Then, download, print and attempt the "Crystal Ball Activity" with other molecules that have not - as yet been asked in the NCEA Level 2 exam
SHAPES OF MOLECULES
NCEA EXAM QUESTION: For each of the molecules shown in the Lewis structures table above
i) draw a 3dimensional (3D) arrangement of the atoms in each molecule
ii) name the shape of each molecule
iii) state the bond angles
Download a Powerpoint "Summary of Level 2 Shapes of molecules"
You may find it useful to download and print the table where there is space provided to draw the 3D shapes
Now, try these!
DESCRIBING AND EXPLAINING SHAPES
NCEA EXAM QUESTION: Describing and Explaining shapes and bond angles. In your answer, you must make reference to the arrangement of electrons.
The Lewis structure for a molecule containing atoms of boron, oxygen, and hydrogen, is shown below.
The following table describes the shapes around two of the atoms in the molecule above. Complete the table with the approximate bond angles
ii) The bond angles x and y in the molecule above are different. Elaborate on why the bond angles are
different. In your answer you should include:
• factors which determine the shape around the:
- B atom for bond angle x
- O atom for bond angle y
• reference to the arrangement of electrons around the B and O atoms
Boron and phosphorus both bond with three fluorine atoms to form BF3 and PF3. However, the molecules have different shapes and bond angles. The following table shows the Lewis structures for the molecules BF3 and PF3.
Explain why these molecules have different shapes and bond angles.
In your answer include:
• the shapes of BF3 and PF3
• factors that determine the shape of each molecule
• the approximate bond angle in BF3 and PF3
• justification of your chosen bond angles for each molecule.
Explain why these molecules have different shapes, but have the same approximate bond angle. In your answer you should include:
• the shapes of SO2 and H2CO
• factors which determine the shape of each molecule
• an explanation of why the approximate bond angle is the same by referring to the arrangement of electrons for each molecule.
The shapes of the two molecules SO2 and H2S are shown in the diagram below. The shape of both molecules is described as bent. Discuss.
Lewis structures for two molecules are given below.
For each molecule, name the shape of the molecule and give a reason for your answer.
The shape of both molecules can be described as bent. However, these molecules do not have the same bond angle.
Discuss why these molecules have different bond angles. Your answer must include:
factors which determine the shape of each molecule
the approximate bond angle for each molecule.
The Lewis structures of the molecules NCl3 and SO3 are given below.
Discuss the shapes and bond angles of these two molecules. For each molecule:
• name the shape
• determine the bond angle
• justify your answers.
Lewis structures for TWO molecules are given below. For each molecule :
• name the shape
• justify your answer.
For each of the molecules in the table, name the shape and explain why it has the shape you have identified.
(i) CH3Cl
(ii) NCl3
(iii) CH2O
Molecules of water (H2O) and ozone (O3) each contain 3 atoms and both the molecules are bent. However, the bond angle in H2O significantly smaller than the bond angle in O3. Discuss.
POLAR BONDS
NCEA EXAM QUESTION: Tick the correct box to state whether the molecules contain a polar bond or non polar bond
POLARITY OF MOLECULES
NCEA EXAM QUESTION: State the polarity of the following molecules
EXPLAINING POLARITY OF MOLECULES
NCEA EXAM QUESTION For each molecule listed in the table above, explain the reason for your choice
Elements M and X form a compound MX2. Atoms of element X have a higher electronegativity value than atoms of element M, therefore the M–X bonds are polar.
Depending on what elements M and X are, molecules of the compound formed will be polar or non-polar.
State the most likely shape(s) of the molecule if it is i) polar or ii) non-polar
Justify your answer and draw diagrams of the possible molecules with dipoles labelled. You do not need to identify what elements M and X are.
NCEA EXAMINATION QUESTION: This question is based on a experiment that you should have carried out in class
...you missed your practical lessons? This virtual experiment may help, click on the image below
Two burettes are set up. One burette contains water (a polar liquid) and the other contains cyclohexane (a non-polar liquid). The liquid is allowed to run from each burette in a steady stream. A charged plastic ruler is then placed near the stream of each liquid.Describe and Explain what will be seen when the charged ruler is placed near the stream of each liquid.
SUMMARY OF BONDING
NCEA EXAM QUESTION: Name the...
i) type of substance
ii) type of particle present in the substance
iii) the bonding (attractive forces between particles)
JUSTIFY PROPERTIES OF SUBSTANCES
NCEA EXAM QUESTIONS: Justify each of the properties of the following substances
Use your knowledge of structure and bonding to explain the dissolving process of sodium chloride in water. Support your answer with an annotated (labelled) diagram.
Silicon dioxide has a melting point of 1770°C.Explain why silicon dioxide has a high melting point by referring to the particles and the forces between the particles in the solid.
see below a copy of a related question that was in theThe diagrams below show structural representations of the two solids ice, H2O, and sodium chloride, NaCl. Ice melts at 0°C and sodium chloride melts at 801°C.
On each diagram above, circle ONE of the forces of attraction which must be overcome for the substance to melt. Give a reason for your choice.
Discuss the reasons why the following two carbon-containing compounds (methane and calcium carbide) have different melting points. The melting points are given in the table below.
Your answer should include:
• the type of particle found in each compound
• the attractive forces found in each compound
• the strength of these attractive forces.
NCEA EXAM QUESTIONS: Compare and contrast the properties of the following substances
Additional questions on Comparing and Contrasting substances
Using your knowledge of structure and bonding, explain why, although both graphite and copper are good conductors of electricity, copper is suitable for electrical wires, but graphite is not.
Energy changes section of the exam paper
NCEA EXAM QUESTIONS: Answer the following questions to Explain Endothermic and Exothermic reactions
a) Explain When solid sodium hydroxide is added to water, the temperature increases.
b) The freezing of water to form ice can be represented by the following equation. H2O(l) → H2O(s)
1) (a) Dissolving ammonium nitrate in a beaker containing water can be represented by the following equation:
NH4NO3(s) → NH4+(aq) + NO3– (aq) ΔrH° = 25.1 kJmol–1
Choose the term (endothermic or exothermic) that best describes this process.
Choose the phrase that best describes what you would observe happening to the beaker during this process (gets colder, stays the same, gets warmer) Explain your choice.(b) Glucose is an important source of energy in our diet. The equation below shows the combustion of glucose to form carbon dioxide and water.C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(ℓ) Δr H° = –2 820 kJ mol–1(i) Choose the term (endothermic or exothermic) that best describes this process. Give a reason for your choice.
(ii) Females who are moderately active need 9 800 kJ of energy per day. Calculate the number of moles of glucose that would provide this daily energy requirement.
(c) Many portable BBQ and camping gas canisters contain butane, C4H10. Butane is a gas at room temperature, and has a boiling point of – 0.5°C. The gas canisters contain both gas and liquid butane. As the gaseous butane is used, some of the liquid evaporates.
Choose the term (endothermic or exothermic) that best describes this process.
Give a reason for your choice, and use your knowledge of structure and bonding, and energy changes, to explain the changes occurring as the liquid evaporates.
a) Some Bunsen burners use methane gas, CH4, as a fuel. The reaction for the combustion of methane in a Bunsen burner is shown in Equation One below.
Equation One: CH4+ 2O2→ CO2+ 2H2O ΔrH = –889 kJ mol–1When this reaction occurs, bonds are broken and bonds are formed. State which bonds are broken and which bonds are formed during the reaction
b)The equation for water boiling at 100°C is shown below in Equation Two.
Equation Two: H2O(ℓ) → H2O(g) ΔrH = 40.7 kJ mol–1Explain why this equation is endothermic. You should relate the energy changes that are occurring to the specific bonds being broken or formed.
a) Ice melting to form water can be represented by the following equation: State whether the reaction is Endothermic or Exothermic and give reasons for your choice
b) The reaction between hydrogen gas and oxygen gas to form water in the gaseous state can be represented by:
When this reaction occurs, bonds are broken and bonds are formed, state which bonds are broken and which bonds are formed. The bond breaking and bond forming processes above can be described as EITHER exothermic OR endothermic. State which process is exothermic and which process is endothermic and Explain your answer
(a) For the reaction 4NH3(g) + 5O2(g) --> 4NO(g) + 6H2O(g), the enthalpy of reaction is ΔrH = –950 kJ mol–1.
The reverse reaction is 4NO(g) + 6H2O(g) --> 4NH3(g) + 5O2(g)
Two energy diagrams are shown below. One is for the forward reaction and one is for the same reaction in the reverse direction.
i) For each diagram, indicate whether the diagram is for an exothermic reaction or an endothermic reaction
ii) For each diagram, indicate whether the diagram is for a forward or reverse reaction
iii) On the diagrams for both reactions, also label the following: • enthalpy of reactionΔrH and • Activation Energy, Ea.
iv) Determine the value for the enthalpy of reaction (ΔrH ) for the reaction shown in Diagram Two. Explain how you determined this value.
NCEA EXAM QUESTIONS: Carry out the following energy change calculations
Methanol and ethanol can both be used as fuels. Their combustion reactions can be represented by the following equations:
Methanol: 2CH3OH + 3O2 → 2CO2 + 4H2O Δr Ho = –1450 kJ mol–1
Ethanol: C2H5OH + 3O2 → 2CO2 + 3H2O Δr Ho = –1370 kJ mol–1
Justify which fuel, methanol or ethanol, will produce more heat energy when 345 g of each fuel is combusted in excess oxygen.
M(CH3OH) = 32.0 g mol–1 M(C2H5OH) = 46.0 g mol–1
1) The equation below shows the combustion of butane. C4H10(g) + 13/2 O2(g) → 4CO2(g) + 5H2O(g)
When 100 g of butane undergoes combustion, 4 960 kJ of energy is released. Calculate the enthalpy change when 1 mole of butane undergoes combustion. M(C4H10) = 58.1 g mol–1.
2) The iron oxides Fe3O4 and Fe2O3 react with aluminium as shown below.
3Fe3O4(s) + 8Al(s) → 4Al2O3(s) + 9Fe(s) Δr H° = –3 348 kJ mol–1
Fe2O3(s) + 2Al(s) → Al2O3(s) + 2Fe(s) Δr H° = –851 kJ mol–1
Justify which iron oxide, Fe3O4 or Fe2O3, will produce more heat energy when 2.00 kg of iron is formed during the reaction with aluminium.
Your answer should include calculations of the heat energy produced for the given mass of iron formed. M(Fe) = 55.9 g mol–1.
1) CH4 + 2O2→ CO2+ 2H2O ΔrH = –889 kJ mol–1 Calculate the energy released when 128 g of methane is burnt. M (CH4) = 16.0 g mol–1.
2)Magnesium burns in oxygen to produce magnesium oxide. The equation for the chemical reaction is be represented by:
2Mg(s)+ O2(g) --> 2MgO(s) ∆rH = –1200 kJ mol–1i) Calculate how much energy is released when 15.4g of oxygen gas reacts.
ii) Calculate the mass of magnesium that must react to release 98.2kJ of energy.
(1) The overall reaction occurring in many disposable hand warmers can be represented by:
4Fe(s) + 3O2(g) → 2Fe2O3(s) ΔrH = –1652 kJ mol–1(i) Calculate the energy released when 1.00 mol Fe2O3 is produced.
(ii) Calculate the mass of Fe that would be required to release 185 kJ of energy. M(Fe) = 55.9 g mol–1.
(2) Hydrogen and methane can be used as fuels.
2H2(g) + O2(g) → 2H2O(ℓ) Δr H = –570 kJ mol–1CH4(g) + 2O2(g) → CO2(g) + 2H2O(ℓ) Δr H = –890 kJ mol–1Determine which of the fuels, hydrogen or methane, provides the most energy per gram of fuel burned. Justify your answer with calculations.
When a 12.2 g sample of ammonia is burned, 275 kJ of energy is released. Calculate the energy released for the reaction below, when four moles of ammonia are burned.
4NH3(g) + 3O2(g) → 2N2(g) + 6H2O(ℓ)The equation for the reaction between calcium oxide, CaO, and water can be represented as Calculate the mass of calcium oxide required to release 287 kJ of energy.
CaO(s) + H2O(ℓ) → Ca(OH)2(aq) ΔH = –82.0 kJ mol–1When an 18.4 g sample of ethanol is burned, 546 kJ of energy is released. Determine the enthalpy change, ΔrH, for the reaction when one mole of ethanol is burned.
C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l)(1) 29.6 g of sodium hydroxide was dissolved in water and excess hydrochloric acid was added. Using the temperature increase and the heat capacity of water, it was calculated that 43.5 kJ of heat was released.
(i) Determine the enthalpy change, ΔrH, for the following reaction:
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)(ii) What mass of sodium hydroxide is required to produce 150 kJ of energy?
(2) The principle of a fireworks-type explosion can be demonstrated by igniting a sucrose jellybaby with sodium chlorate, NaClO3. The equation for the explosion reaction is:
2NaClO3(s) + C12H22O11(s) → 2NaCl(s) + 9C(s) + 3CO2(g) + 11H2O(g) Δr H° = –2192 kJ mol–1Calculate...
(i) the quantity of heat released when one jelly-baby containing 4.56 g of sucrose (C12H22O11) is exploded.
(ii) The heat released by the explosion can be used to vaporise strontium chloride to give the fireworks colour.
(iii) The heat required to convert SrCl2 from the solid to the gas state is 343 kJ mol–1.
(iv) Use your answer to (i) above to calculate the mass of solid SrCl2 that can be vaporised by exploding one jelly baby containing 4.5 g of sucrose.
Octane is a key component in petrol, and burns according to the following equation:
C8H18(l) + 12 1/2 O2(g) → 8 CO2 (g) + 9 H2O(l) ΔrH = -5500 kJ mol-1(i) 1.00 litre of octane contains 6.12 moles of the fuel. Calculate the energy released when 1 litre of the fuel is burnt.
Using hydrogen gas (H2) as a fuel for cars, rather than octane, is often viewed as better for the environment.
(ii) Calculate the mass of H2 required to produce the same amount of energy as 1.00 litre of octane, as calculated in part (b) above. State your answer to 3 significant figures. H2(g) + 1/2 O2(g) → H2O(g) ΔrH= -286 kJmol-1
BOND ENTHALPY
NCEA EXAM QUESTIONS: Carry out the following Bond Energy Calculations
Use the Bond Enthalpy values provided in the table below to answer the questions (referenced from previous NCEA Exams)Hydrogen gas, H2(g), reacts with oxygen gas, O2(g), as shown by the following equationH2(g) + ½ O2(g) → H2O(g) Δr Ho = –242 kJ mol–1Given the average bond enthalpies in the table below, calculate the average bond enthalpy of the O – H bond in H2O.
Chlorine reacts with methane to form chloromethane and hydrogen chloride, as shown in the equation below.
CH4(g) + Cl2(g) → CH3Cl(g) + HCl(g)Use the following bond enthalpies to calculate Δr H° for this reaction.
a)The equation for the combustion of propan-1-ol is:
CH3CH2CH2OH(g) + 4½O2(g) → 3CO2(g) + 4H2O(g) ΔrH = –2010 kJ mol–1Calculate the bond enthalpy for the C=O bond, using the enthalpy of the reaction above and the bond enthalpy data
b) Define bond enthalpy and explain why the bond enthalpy value calculated for C=O is higher than the C–O bond enthalpy.
Complete combustion of methanol can be represented by the following equation: Use the bond enthalpies to calculate ΔrH for this reaction.
2CH3OH(g) + 3O2(g) → 2CO2(g) + 4H2O(g)The equation for the combustion of ethanol is: CH3CH2OH(g) + 3O2(g) → 2CO2(g) + 3H2O(g) ΔrH° = –1379 kJ mol–1
Calculate the bond enthalpy for the O–H bond using the enthalpy of the reaction above and the bond enthalpy data in the table above
Calculate the enthalpy change for the reaction below using the bond enthalpy data in the table.
CH3Cl(g) + NH3(g) → CH3NH2(g) + HCl(g)Methylhydrazine, N2H3CH3, can be used as a fuel. The structural formula for methylhydrazine is
ii) Use the bond enthalpies given in the table below to calculate the energy released when one mole of methylhydrazine vapour is burned.
N2H3CH3(g) + 2½O2(g) → CO2(g) + N2(g) + 3H2O(g)Calculate the enthalpy of formation of water in the gas state, ΔfH°(H2O, g)
Carbon monoxide is reacted with steam to produce hydrogen gas.
CO(g) + H2O(g) --> H2(g) + CO2(g) ΔrH = – 41.2 kJ mol–1The bond enthalpies for the carbon to oxygen bonds in CO2 and CO are different.
i) Use the bond enthalpies and the enthalpy of the reaction to calculate the bond enthalpy of the carbon to oxygen bond in CO. Why are bond enthalpy values always positive? Explain the difference between the following bond enthalpies.
ii) Why are bond enthalpy values always positive?
iii) Explain the difference between the C – O and C = O enthalpies
A fuel cell, such as that used on a space-craft, is similar to a battery. An example is the fuel cell that ‘burns’ hydrogen and oxygen to produce water and energy.
The overall equation for the reaction is
2H2(g) + O2(g) --> 2H2O(l ) ΔrH ° = – 572 kJ mol–1a) If the water produced is in the gas phase the equation for the reaction is
2H2(g) + O2(g) --> 2H2O(g)b) Use the bond enthalpies to calculate ΔrH ° for this reaction. Write an equation for which the enthalpy change is equal to Δ vapH ° (H2O).
c) By considering the nature of the reaction in part (b), describe why it is an endothermic change.
d) Using the information above, calculate the value of Δ vapH ° (H2O).
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